My betta has been happily living in his 2 gallon hex tank for about a month with no issues. He'd just been getting over fin rot and his tail had healed exceedingly well! His tail was still slightly ragged, but definitely on the mend. Last night he was looking a little pale, but nothing very unusual. I figured I should just closely monitor his appearance. Well, I woke up and he was dead ;_; What could it have been?

His tank was heated at 80 degrees.
Decor included two silk realistic plants, gravel, and his thermometer.
I use seachem prime dechlorinator.
I performed 100% water changes every other day. (monday, wednesday, friday, etc)
He's always been really energetic and eating well. No symptoms whatsoever until last night, and then death by morning.